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Predictability of tropical rainfall in the ECMWF seasonal forecast systems

Predictability of tropical rainfall in the ECMWF seasonal forecast systems. Franco Molteni, Tim Stockdale, Magdalena Balmaseda, Laura Ferranti, Kristian Mogensen, Frederic Vitart ECMWF, Reading, U.K. The new ECMWF Seasonal fc. system (Sys-4). IFS 36R4 0.7 deg (T255) 91 levels. NEMO

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Predictability of tropical rainfall in the ECMWF seasonal forecast systems

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  1. Predictability of tropical rainfall in the ECMWF seasonal forecast systems Franco Molteni, Tim Stockdale, Magdalena Balmaseda, Laura Ferranti, Kristian Mogensen, Frederic Vitart ECMWF, Reading, U.K.

  2. The new ECMWF Seasonal fc. system (Sys-4) IFS 36R4 0.7 deg (T255) 91 levels NEMO 1/1-0.3 d. lon/lat 42 levels OASIS-3 H-TESSEL Ens. Forecasts Initial Con. 4-D variational d.a. Gen. of Perturb. System-4 CGCM 3-D v.d.a. (NEMOVAR)

  3. ECMWF System 4: main features • Operational forecasts • 51-member ensemble from 1st day of the month • released on the 8th • 7-month integration • Experimental ENSO outlook • 13-month extension from 1st Feb/May/Aug/Nov • 15-member ensemble • Re-forecast set • 30 years, start dates from 1 Jan 1981 to 1 Dec 2010 • 15-member ensembles, 7-month integrations • 13-month extension from 1st Feb/May/Aug/Nov

  4. Bias in S4 re-forecasts: SST (DJF) Start: 1 Nov. 1981/2010 Verify: Dec-Feb System 4 System 3

  5. Bias in S4 re-forecasts: rainfall (JJA) Start: 1 May 1981/2010 Verify: Jun-Aug System 4 System 3

  6. Ens-mean ACC in S4 re-forecasts: rainfall (JJA) Start: 1 May 1981/2010 Verify: Jun-Aug System 4 System 3

  7. Validation of leading rainfall EOF/PC in 8 regions • Comparison of model EOF patterns vs. 1st EOF of GPCP 2.1 • Prediction of the interannual variability of PC1 using 1-to-3-dim. model EOF subspaces • Comparison of actual vs. perfect-model correlation skill for PC1

  8. Space and time correlations for rainfall EOF1/PC1 Region EOF-1 PC-1

  9. Variability of tropical rainfall: EOF comparison

  10. Variability of tropical rainfall: EOF comparison

  11. Variability of tropical rainfall: EOF comparison

  12. Variability of tropical rainfall: EOF comparison

  13. Actual vs. perfect-model skill: rainfall PC-1

  14. Summary • New ECMWF seasonal fc. System-4 (S4): IFS-NEMO coupled model, 3-D var. ocean data assimilation (NEMOVAR), higher atmos. spatial resolution than S3, larger ensemble size, extended re-forecast set. • Rainfall biases in S4: reduced rainfall biases overall, but with persistent positive bias over the Maritime Continents; consistent with too strong Walker circulation in the West Pacific and related SST biases. • Rainfall forecast skill: S4 similar to S3 in the tropical Pacific, improved over the tropical Atlantic and Indian Oceans, still noisy and often marginal over the continents when grid-point data are considered. • Leading EOFs of rainfall variability: more realistic patterns of rainfall variability in S4, especially over South America and Africa. Incorrect teleconnection between West Pacific and the Indian subcontinent. Prediction of GPCP PC1 exceeds 50% correlation over all tropical regions. • Reliability: the enhanced internal variability and better match between spread and error lead to more reliable seasonal forecasts w.r.t. S3 in both tropical and extra-tropical regions.

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